]60 SLEEP OF PLANTS. 



are found most affected by light, insomuch that it 

 appears, in several cases, the sole cause of their expan- 

 sion, for when it is withdrawn they fold over each 

 other, or droop, as if dying ; and this is called by 

 Linnasus the Sleep of Plants, who has a dissertation on 

 the subject in his Amcenitates Academics. The term 

 Sleep may not really be so hyperbolical as at first sight 

 it seems; for the cessation of the stimulus of light, and 

 of the consequent restrained position of the leaves, may 

 be useful to the vegetable constitution, as real sleep is 

 to the animal. Another purpose is answered by the 

 nocturnal folding of some leaves, that they shelter their 

 flowers from the dew, the advantage of which we shall 

 explain hereafter. 



Some pinnated leaves display a more extraordinary 

 sensibility, not merely to light, but to the touch of any 

 extraneous body, or to any sudden concussion, as those 

 of Mimosa sensitive*, and pudica, Oxalis sensitiva, 

 and Smithia sensitiva, Ait. Hort. Kew. v. 3. t. 13. 

 An impression made even in the most gentle manner, 

 upon one of their leaflets, is communicated in succes- 

 sion to all of them, evincing an exquisite irritability, 

 for it is in vain to attempt any mechanical solution of 

 this phenomenon. One of this tribe, Hedysarum 

 gyrans, has a spontaneous motion in its leaves, inde- 

 pendent of any external stimulus, even of light, and 

 only requiring a very warm still atmosphere to be per- 

 formed in perfection. Each leaf is ternate, and the 

 small lateral leaflets are frequently moving up and 



